Claude on Question 7, Sep 22

A refrigerator does not actually make cold; it moves heat from the inside of the cabinet to the room around it. The trick that makes this possible is a fluid called a refrigerant, which is chosen because it boils and condenses at convenient temperatures and pressures. By repeatedly changing this fluid between liquid and gas, the appliance can pick up heat where you want it cold and dump it somewhere else.

The cycle begins at the compressor, the small motorized pump you can usually hear humming at the back or bottom of the unit. It takes in low-pressure refrigerant vapor and squeezes it, which raises both its pressure and its temperature so that the gas is now hotter than the kitchen air. This hot, high-pressure gas travels into the condenser, a network of thin tubes, often with fins, mounted on the back or underneath the refrigerator. As air flows over these tubes, the refrigerant gives up its heat to the room and condenses into a warm liquid, still under high pressure. This is why the space behind a working refrigerator feels warm and why a room gets slightly hotter, not cooler, if you leave the door open.

From there the liquid passes through a narrow restriction, either a capillary tube or an expansion valve, which throttles the flow and causes a sudden drop in pressure. Lowering the pressure lowers the temperature at which the refrigerant boils, so on the far side of the restriction the fluid becomes a very cold mixture of liquid and vapor. It then enters the evaporator, the coils tucked inside the freezer compartment or behind an inner panel. Here the refrigerant absorbs heat from the food and air inside the cabinet, which it needs in order to boil, and that absorption of heat is what actually chills the interior. Having turned fully into a cool low-pressure gas, the refrigerant returns to the compressor and the whole sequence repeats.

Several supporting parts make the system practical. Thick foam insulation and rubber door gaskets slow the leakage of heat back inside, so the compressor need not run constantly. A thermostat or electronic sensor measures the interior temperature and switches the compressor on and off to hold it near the set point. A small fan usually circulates air over the evaporator and distributes the cold through the freezer and the fresh food compartment, with a damper or separate cooling circuit keeping the two at different temperatures. Because moisture from the air freezes onto the cold evaporator, most modern units also include a heater and timer that periodically melt the frost, sending the meltwater down a tube to a shallow pan near the condenser, where the waste heat from the coils evaporates it away. The energy to drive all of this comes from the electricity powering the compressor, which is the price paid for pushing heat from a cold place to a warmer one, something that will never happen on its own.